This invention relates generally to support systems used in the construction industry, and more particularly to ladder-type cable tray used to support runs of cable, electrical wiring and the like.
Conventional ladder-type cable tray comprises a pair of parallel side rails and a series of spaced-apart rungs extending between the rails, the rails and rungs typically being of extruded aluminum (although they may also be steel parts). It has been the conventional practice in the industry to secure the rungs to the rails by welding. Welding, however, has certain disadvantages, including inconsistencies in the weld joints, the creation of noxious fumes, the need for protective equipment, high labor and capital costs, a relatively low production rate, and the need to clean the parts after the welding is complete.
Attempts have been made to fasten the rungs to the rails by screw fasteners. These attempts have involved fabricating the rungs with C-shaped or U-shaped screw holes in the ends of the rungs, and driving self-tapping screws through the side rails into these screw holes to fasten the rungs to the rails. In these prior designs, the screw holes are configured to be open along one of their sides to permit expansion of the holes to accommodate varying screw and rung tolerances. However, this open design also reduces the extent to which the screws may be tightened, which decreases the overall mechanical strength of the cable tray. Further, when the cable tray deflects under a load, the screws tend to pop through the open sides of the screw holes, which reduces the load-bearing capacity of the cable tray.
There is a need, therefore, for ladder-type cable tray which eliminates the problems associated with welding the rungs to the side rails, but which has the strength characteristics of welded cable tray.